Hawes Mechanical Television Archive by James T. Hawes, AA9DT
Mechanical Television Camera Experiments, Part 1

Almost a Time Machine

New dimension. Mechanical TV is almost like stepping into a time machine: A time machine that we've built with our own hands and minds. Mechanical TV from our own camera adds a new dimension to the experience. Now, we can take and share pictures, test different circuits and learn more with every improvement.

Block diagram of 
        mechanical TV camera (mechanisches Fernsehen mit Kamera). 
        Makes great science fair project!

The Pickup

Pickup Block. In the diagram above, Block 1 is our pickup device. For Block 1, many experimenters use a small solar cell or photodiode. Solar cells can pick up a fairly broad spectrum of light. If you use a solar cell, stick with a small one. Larger cells respond too slowly for this project. Photodiodes respond very quickly to light changes. Unfortunately, photodiodes require extra amplification.


Easy to use. My personal preference for Block 1 is a phototransistor. It's not as quick as a photodiode, but it offers some gain. Also, a phototransistor is more durable and easier to work with than a solar cell. See the phototransistor circuits nearby. You can use many types of phototransistors in these circuits. Two that I've used recently are...

  • Texas Instruments TIL414 from RadioShack

  • Ledtech LT9593-91-0125 from All Electronics

Schematic: Phototransistor, 
         mechanical (mechanisches Fernsehen) video pickup circuit
High-gain phototransistor circuit
for mechanical video pickup
Requirement: Clear case. Make sure that you use a phototransistor with a clear, plastic envelope or case. Types with black cases won't work for this project. A black case blocks visible light. Some infrared phototransistors have these black (or dark color) cases. Other devices have a clear case, and respond to both infrared and visible light. These dual-purpose types are fine. Radio Shack, Mouser, Digi-Key and All Electronics carry suitable phototransistors.

Install the phototransistor in a rectangular project box. Bud® makes suitable boxes. You'll find such boxes at Radio Shack, Mouser, and other vendors. The box must have plenty of room for your pickup and a preamp. Allow a couple of extra inches between the end of the box and the pickup cell. Of course, the box must not be too large to fit inside your scanner. The light pickup will face a window that you'll cut in one short side of the box. This side becomes the front of the box. In a moment, I'll give more details about the window. You'll mount your PC board in the back of the box.

Schematic: Mechanical video (mechanisches Fernsehen) 
         phototransistor circuit with faster response speed
Circuit with faster response
speed, but voltage gain of 0.9

The window should be about the size of our video picture frame, or slightly larger. Mount the phototransistor about two inches behind the window. The window and phototransistor must face the Nipkow disc apertures.

The Preamplifier

Preamplifier blocks 2 and 3 tend to be difficult for beginners to design and build. If you're determined, curious or persistent, forge ahead anyway. Beginners often surprise themselves with their success.

How do we design a phototransistor preamplifier? We start by examining circuits. Maybe we can use or modify something that already exists. There are lots of circuits, and many are easy to build. Below is one idea that might help...

Low-level preamp. Our low-level preamplifier Q2 is an NPN, common-emitter stage. Our phototransistor is Q1. The Q1 circuit is an emitter-follower stage. We direct couple this stage to Q2. Output signal Vo comes off Q2 between collector and ground. Transistor Q2 is a general-purpose, silicon, small-signal type. For example, 2N2222, 2N3904 or 2N4401.

(Hfe: 200 or above. Germanium types won't work in this circuit.)

       diagram of low-level preamplifier
Low-level preamplifier improves
phototransistor sensitivity

Limits. The signal is still a high-impedance one. Yet we have a much stronger signal than we started with. At 1 kHz, Q2's unloaded gain is 179. Unfortunately, this is a pie-in-the-sky gain figure. When we connect Q2 to anything meaningful, our gain crashes back toward earth.

Capacitors. You'll notice the large decoupling capacitor. We need Paul Bunyonesque capacitors so that we can reproduce frequencies down to nearly DC. These frequencies account for crucial large picture details.

Need: More Gain! Our new circuit is helpful for experiments. Yet we probably still need more voltage gain. I suspect that our solution requires two or three more transistors. On the next page, I explain how to add transistor stages. Believe it or not, there's a very easy way to do that!

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WARNING. This is your project. Your achievement is entirely yours. I assume no responsibility for your success in using methods on these pages. If you fail, the same is true. I neither make nor imply any warranty. I don't guarantee the accuracy or effectiveness of these methods. Parts, skill and assembly methods vary. So will your results. Proceed at your own risk.

WARNING. Electronic projects can pose hazards. Soldering irons can burn you. Chassis paint and solder are poisons. Even with battery projects, wiring mistakes can start fires. If the schematic and description on this page baffle you, this project is too advanced. Try something else. Again, damages, injuries and errors are your responsibility. — The Webmaster

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